How to dynamically adjust the length of a message based on the second byte?

[imso]

Assuming my second byte in hex is a representation of the message length, how can I dynamically adjust the length of the message the user can key into the textbox based on the hex value of the second byte?

Message Byte entries into textbox: xx xx xx xx xx xx xx xx xx xx xx xx ..... where x is 0-9, a-f in hex.

The code below was used as a sort of my goal keeper to allow only hex entry into the textbox but I want to further enhance the code below with the additional function mention above? Sorry if I couldn't explain it clearly enough due to my inadequate English command. Thanks

void textBox1_TextChanged(object sender, EventArgs e)
{
    int caret = textBox1.SelectionStart;
    bool atEnd = caret == textBox1.TextLength;
    textBox1.Text = sanitiseText(textBox1.Text);
    textBox1.SelectionLength = 0;
    textBox1.SelectionStart = atEnd ? textBox1.TextLength : caret;
}

void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (!isHexDigit(e.KeyChar) && e.KeyChar != '\b')
        e.Handled = true;
}

string sanitiseText(string text)
{
    char[] result = new char[text.Length*2];

    int n = 0;

    foreach (char c in text)
    {
        if ((n%3) == 2)
            result[n++] = ' ';

        if (isHexDigit(c))
            result[n++] = c;
    }

    return new string(result, 0,  n);
}

bool isHexDigit(char c)
{
    return "0123456789abcdef".Contains(char.ToLower(c));
}